1. Field of the Invention
The present invention relates to the field of mass spectrometry, and more particularly to system that includes a novel internal volume (e.g., a continuous flow guide) configuration that provides for an optimal flow path within a mass spectrometer ion source housing.
2. Discussion of the Related Art
In LC-MS mass spectrometry analyte, ions are generated by nebulizing the ion-containing liquid at pressures around atmospheric in a chamber called the ion source housing. Some of this vapor enters the mass spectrometer's vacuum chamber for analysis. The remainder of the vapor, as well as the gases aiding the nebulizing and desolvation processes (sheath and auxiliary) need to exit the source housing through a different opening called a drain. To prevent contamination of surfaces in the source housing which can cause interference with the analysis of subsequent samples, so called carry-over, it is important that the vapor-laden gases are removed swiftly.
A process that often occurs in conventional housings is back streaming/back drafting, wherein a counter flow of gases out of the drain region are caused by the flow of the high velocity nebulizing and desolvation gases directed towards the drain. This counter flow contains undesired contaminants and can cause stagnation of the vapor-laden gases which can lead to condensation of droplets on source surfaces and therefore contamination of such surfaces, which can lead to carryover. In particular, analyte circulating within an ion source housing creates a memory effect wherein the intensity of the analyte as read by a configured mass analyzer can be seen to decay over a period of time after introduction of the analyte/solvent mixture into the housing has ceased. Accordingly, if a second analyte is introduced too soon after the first analyte, the first analyte can still appear in a subsequent reading. This negatively impacts the performance of the mass spectrometer as expressed in the number of samples that can be analyzed per hour. Moreover, the carryover of analytes can sometimes lead to their re-entry into the chamber atmosphere and contribute to a background that reduces the signal-to-noise ratio of desired mass spectral runs.
Background information on a system and method that attempts to address the aforementioned problems through the use of an inner exhaust tube is described and claimed in U.S. Pat. No. 6,759,650, entitled, METHOD OF AND APPARATUS FOR IONIZING AN ANALYTE AND ION SOURCE PROBE FOR USE THEREIN,” issued Jul. 6, 2004, to Covey et al, including the following, “Ions for analysis are formed from a liquid sample comprising an analyte in a solvent liquid by directing the liquid sample through a capillary tube having a free end so as to form a first flow comprising a spray of droplets of the liquid sample, to promote vaporization of the solvent liquid. An orifice member is spaced from the free-end of the capillary tube and has an orifice therein. An electric field is generated between the free-end of the capillary and the orifice member, thereby causing the droplets to be charged, and the first flow is directed in a first direction along the axis of the capillary tube. Two gas sources, or an arc jet of gas, provide second and third flows, of a gas, and include heaters for heating the second and third flows. The second and third flows intersect with the first flow at a selected mixing region, to promote turbulent mixing of the first, second and third flows, the first, second and third directions being different from one another, and each of the second and third directions being selected to provide each of the second and third flows with a velocity component in the first direction and a velocity component towards the axis of the capillary tube, thereby to promote entrainment of the heated gas in the spray, with the heated gas acting to assist the evaporation of the droplets to release ions therefrom. At least some of the ions produced from the droplets are drawn through the orifice for analysis.”
Background information on a corresponding system and method that attempts to address the aforementioned problems also through the use of an inner exhaust tube is described and claimed in PCT Application No. WO 2008/124264 A2, entitled, DEVICE, APPARATUS AND METHODS FOR MASS SPECTROMETRY,” published Oct. 16, 2008, to Tomany et al, including the following, “The invention comprises apparatus for use with atmospheric pressure ionization sources in which an aerosol is formed from a solution of a sample. The aerosol is received in a hollow member and discharged outside the chamber of the ionization source in order to reduce contamination of the ionization source itself by involatile material in the solution and by previously analysed samples. The hollow member is easily removable from the ionization source to facilitate cleaning and replacement. Ionization sources, mass spectrometers, and ion mobility spectrometers comprising the apparatus are also described.”
Background information on a system and method that minimizes recirculation problems is described and claimed in U.S. Pat. No. 7,145,138, B1 entitled, EXHAUST PORT DESIGN FOR API SOURCES,” issued Dec. 5, 2006, to Rohan A. Thakur, including the following, “The present invention provides Atmospheric Pressure Ionization (API) sources that minimize recirculation of droplets, solvent, and background gas. The APIs of the present invention comprise a chamber for ionizing samples for mass spectroscopy and similar analytical equipment. A spray probe on one side of the chamber directs a spray of a solvent and a sample as a cone of droplets that pass proximate to an ion exit orifice. Ions from the spray cone are extracted through the ion exit orifice. An exhaust port is disposed opposite the spray probe and aligned therewith to collect the spray cone. The design of the exhaust port, as described below, minimizes recirculation within the chamber to reduce the memory effect and to preserve the signal to noise ratio.”
Therefore, a need exists for an improved source housing in which a novel internal volume (e.g., a continuous flow guide) has been designed to have an optimal flow path so as to enable a user to easily clean the entire ion source housing/internal volume apparatus and minimize undesired carryover. The present invention is directed to such a need.